In the manufacture of dry silver emulsions, it is generally preferred to work with half soap emulsions. These are emulsions which constitute less than 100% by weight of the silver salt of an organic acid, and the remainder up to 100% being constituted by the organic acid. In the conversion of an organic acid to a full soap (100% silver salt of the acid), the solubility of the composition changes, and different solvents or conditions must be used to keep the material in solution or wetted by the solution. This change in solvent requirements often occurs at about 90% conversion of an alkali or ammonia salt of the acid to the silver salt of the acid.
U.S. Pat. Nos. 3,839,049 and 3,761,273 disclose the use of preformed silver halides in the formation of dry silver emulsions and elements. In these references, significant loss of silver (greater than 10% loss) is noted in the conversion to the silver salt. High pH solutions are used in U.S. Pat. No. 3,761,273 which tend to form silver hydroxide which contributes to silver loss and emulsion inefficiency. When silver halide is added or formed in such high pH solutions, as in the teachings of the above-identified patents, a black precipitate of silver hydroxide tends to form, reducing the performance of the dry silver emulsion.
U.S. Pat. No. 3,839,049 lowers the temperature of solutions containing the alkali metal or ammonia salts of the acid so as to produce colloidal dispersions in an effort to control particle size of the finished emulsion. This causes increased viscosity making the dispersion difficult to work with and contributes to the loss of silver in the process which (in the form of silver nitrate) cannot react with the behenic acid salt in the interior of the dispersed particles.
U.S. patent application Ser. No. 803,549, filed June 6, 1977, also discloses a process for forming photothermographic emulsions with preformed or coprecipitated silver halide crystals. The process disclosed therein also allows the pH of the dispersion to rise above 9.0 when converting the alkali metal or ammonia salt of the organic acid to a silver salt in the presence of silver halide. This also reduces the yield of the silver salt and reduces the purity of the emulsion by forming silver hydroxide during the process.